1. Introduction
This invention relates to polymers and hydrogels of polymers from alkyl acrylates or methacrylates, dihydroxyalkyl acrylates or methacrylates and epoxidized acrylates or methacrylates. The hydrogels are particularly useful for the formation of contact lenses.
2. Description of the Prior Art
As is known in the art, contact lenses are frequently made from polymethyl methacrylate. Such lenses are known as the "hard lenses". Many people cannot adapt to the presence of the hard lens in the eye and with other, the lens compromises the physiological processes required for corneal metabolism. For many, minor irritations are caused by small particles that become lodged under the hard lens resulting in irritation of the cornea. Moreover, it has been found that after wearing a hard lens for an extended time -- e.g., from one to five years, many people experience discomfort and are forced to discontinue its use.
In view of the above difficulties, an effort has been made to develop new lens materials which overcome the problems of the hard lens. One class of such lens materials is described in U. S. Pat. Nos. 2,976,576 and 3,220,960 incorporated herein by reference. These materials are hydrogels of a sparingly cross-linked hydrophilic copolymer, of a major amount of a monoester of an olefinic acid selected from the group of acrylic and methacrylic acids having a single olefinic double bond and a minor amount of a polymerizable diester of one of said acids, the diester having at least two olefinic double bonds. The copolymer is formed by copolymerization in a solvent medium.
One specific hydrogel disclosed in the aforesaid patents is a slightly cross-linked material comprising a predominant quantity of 2-hydroxyethyl methacrylate. This hydrogel, known as "Hema" is used for contact lens fabrication in part, because of its ability to absorb water of hydration, typically from about 35 to 65 percent by weight. The water renders the lens flexible and soft, which enables it to mold to the curvature of the eye. This is in contrast to the conventional hard lens which maintains a rigid configuration that does not always conform to the eye's curvature.
An advantage of the Hema lens is that with proper design, it can be worn with almost immediate comfort. Another advantage is that relative to the hard lens, the difficulties resulting from particles lodging beneath the lens irritating the cornea is reduced.
Despite the above advantages, there are problems that have prevented universal acceptance of the Hema lens. One problem involves a lack of clarity of central vision. For many, the Hema lens does not give steady vision because a constantly changing optical surface results from eye movement and blinking. A second problem pertains to astigmatism. Hard lenses correct astigmatism by providing a new surface on the cornea. Because of the flexibility of the Hema lens, it conforms to the shape of the eye and frequently does not provide the new surface necessary to correct astigmatism. Other difficulties are experienced with use of the Hema lens such as corneal irritation and folds in the membrane of the eye. The exact causes are not known though it has been reported that lacrimal interchange with the Hema lens is minimal compared to the hard lens, possibly due to lens conformation to the contour of the eye which prevents flow of lacrimal fluid beneath the edges of the lens. A reduction in lacrimal fluid reduces the flow of oxygen to the cornea and relief from an accumulation of catabolic products. Finally, the Hema lens has been reported to tear easily.
As will be discussed in greater detail below, the subject hereof is new hydrogel having utility including use as a soft contact lens. The hydrogel is formed from a dihydroxyalkyl acrylate or methacrylate, an alkyl acrylate or a methacrylate, and preferably, a minor amount of an epoxidized alkyl acrylate or methacrylate. The hydrogel is formed by bulk polymerization and is characterized by insolubility in water and many solvents. The preferred monomers are glyceryl methacrylate, methyl methacrylate and glycidyl methacrylate within a given range of ratios. Copolymers that are solvent soluble, but water insoluble formed from glyceryl methacrylate and methyl methacrylate are known and described by H. Yasuda, C. E. Lamaza, and L. D. Ikenberry, Makromol. Chem. 118, 1935 (1968) and H. Yasuda, C. E. Lamaza, and A. Peterlin, J. Polym. Sc. Part A-2, 996, 1,117-1,131 (1971).
The copolymers described in the first of these publications are polymerized in a 70/30 acetic acid/water solvent system. The solutions are prepared with 5% by weight total monomer dissolved in 95% by weight solvent. After eight to ten days at room temperature, precipitation of the polymer is effected in water. Six copolymers are described having monomer mole ratios of methyl methacrylate and glyceryl methacrylate of from 95:5 to 70:30.
The procedure of the second publication is similar to that of the first except that the reaction is carried out in a nonaqueous solvent. For reasons below, the copolymers of these publications differ markedly from the hydrogels described herein and do not have properties suitable for formation of a contact lens.